Stiffness and elasticity of CL materials

The correct answer is C. Strength specifically relates to the force needed to actually break rather than deform a material.

2 Which of the following describes the modulus of a lens material? (A) The resistance to flexure; (B) The resistance to tension; (C) The force per unit area needed to cause failure; (D) The force per unit area to deform a material.
The correct answer is D. Unlike strength, the modulus refers to the force needed to deform a material.

3 Which of the following is equivalent to 1 Pa (Pascal)? (A) 1 Nm-2; B 106 Nm-2; (C) 109 Nm-2; (D) 1 dyn cm-2.
The correct answer is A. The Pascal is the Systeme Internationale unit for modulus and is defined in Newtons per square metre.

4 Which of the following describes the resistance of a material to penetration? (A) Toughness; (B) Hardness; (C) Modulus; (D) Strength.
The correct answer is B. Hardness is specifically used to describe the resistance a material offers to penetration.

5 How does rigidity modulus differ from Young's modulus? (A) The rigidity modulus is only for hard materials; (B) Rigidity relates to compressive stress, Young's to tensile stress; (C) Young's relates to flexure, rigidity to indentation; (D) They are synonymous.
The correct answer is B. Rigidity is specific to deformation by indentation whereas Young's modulus describes deformation under tension and stretching.

6 Which of the following might represent the compressive stress exerted by the lid on the cornea (in 104 dyn cm-2)? (A) 1.2; (B) 2.0; (C) 2.6; (D) 9.
The correct answer is C. Micro-indentation using a flattened indenter has found a modulus with stress relating to that due to the action of the lids on the cornea of 2.6 x 104 dyn cm-2.

7 How does the Young's modulus for a viscoelastic material differ from that for a truly elastic material? (A) The value will be higher for a viscoelastic material; (B) The value will be constant for a truly elastic material; (C) The value will be constant for a viscoelastic material; (D) There is no difference.
The correct answer is B. A truly elastic material will deform at a rate directly related to the force over area as represented by a straight line gradient. A viscoelastic material, however, changes property as it deforms and hence the Young's modulus (or gradient of the graph) will change.

8 Which of the following may be related to a low Young's modulus value? (A) Poor handling; (B) Poor ability to mould to ocular contour; (C) Scratch resistance; (D) Low oxygen transmissibility.
The correct answer is A. A low Young's modulus will result in a very floppy lens, making handling less easy.

9 Which of the following materials has the highest Young's modulus value? (A) Balafilcon A; (B) Lotrafilcon A; (C) Etafilcon A; (D) Galyfilcon A.
The correct answer is B. Lotrafilcon A has a modulus of 120g mm-2.

10 Which of the following signs is unlikely to be mechanical in origin? (A) SEAL; (B) CLPC; (C) Neovascularisation; (D) Lacrimation.
The correct answer is C. New vessel growth is likely to be a response to hypoxic conditions or subsequent to severe inflammatory response.

11 Which is the most likely location to find a SEAL? (A) Central cornea; (B) Superior cornea 1mm from the limbus; (C) Superior cornea 3mm from the limbus; (D) Interpalpebral zone.
The correct answer is B. As the name suggests (superior epithelial arcuate lesion), a SEAL is most likely to be found around 1mm from the superior limbus.

12 Which of the following is not used in the management of CLPC? (A) Topical lubricants; (B) Considering lower modulus lens material; (C) Mast cell stabiliser; (D) Topical decongestant.
The correct answer is D. A topical decongestant may reduce redness resulting from ocular irritation, but is not an effective tool in the management of papillary conjunctivitis.